Microbial Metabolism, Growth, and Control: Study Guide for Exam 2

Basic Metabolism & Biochemistry

Redox Reactions and Metabolic Concepts

• Oxidation: The loss of electrons from a molecule, atom, or ion. In biological systems, this often involves the removal of hydrogen atoms.

• Reduction: The gain of electrons by a molecule, atom, or ion. In cells, this often involves the addition of hydrogen atoms.

• Redox Reactions: Chemical reactions involving the transfer of electrons between two species. Oxidation and reduction always occur together.

• Examples:

  • NADH \( \rightarrow \) NAD+ (oxidation of NADH)

  • ADP + Pi + Energy \( \rightarrow \) ATP (phosphorylation, energy storage)

• Metabolism: The sum of all chemical reactions in a cell, divided into:

  • Anabolism: Biosynthetic reactions that build complex molecules from simpler ones (require energy).

  • Catabolism: Degradative reactions that break down complex molecules into simpler ones (release energy).

Enzymes & Catalysis

Structure, Function, and Regulation of Enzymes

• Enzymes: Biological catalysts, usually proteins, that speed up chemical reactions without being consumed.

• Genetic Basis: Enzymes are encoded by genes (DNA sequences).

• Active Site: The region on the enzyme where the substrate binds and the reaction occurs.

• Competitive Inhibition: Inhibitor molecule competes with the substrate for binding at the active site, reducing enzyme activity.

• Factors Affecting Enzyme Activity:

  • Temperature: High or low temperatures can decrease activity; extreme heat can cause denaturation.

  • pH: Each enzyme has an optimal pH; deviations can reduce activity or denature the enzyme.

  • Substrate Concentration: Increased substrate increases reaction rate up to a saturation point.

  • Inhibitors: Chemicals that decrease enzyme activity (competitive or noncompetitive).

• Denaturation: Loss of protein structure (and function) due to extreme conditions.

• Mesophiles vs Thermophiles:

  • Mesophiles: Grow best at moderate temperatures (20–45°C).

  • Thermophiles: Grow best at high temperatures (45–80°C).

Cellular Energy & ATP Production

Electron Carriers and ATP Synthesis

• Electron Carriers: Molecules like NADH and FADH2 that transport electrons during cellular respiration.

• Electron Transport Chain (ETC): Series of membrane proteins that transfer electrons, releasing energy to pump protons and generate ATP.

• Proton Motive Force (PMF): Electrochemical gradient of protons across a membrane, used to drive ATP synthesis.

• Oxygen as Final Electron Acceptor: In aerobic respiration, oxygen accepts electrons at the end of the ETC, forming water.

• ATP Synthase: Enzyme that synthesizes ATP from ADP and Pi, powered by the flow of protons down their gradient.

Metabolic Pathways

Major Pathways of Energy Production

• Glycolysis: Breakdown of glucose to pyruvate, producing ATP and NADH.

• Citric Acid Cycle (Krebs Cycle): Oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP.

• Oxidative Phosphorylation: Uses electrons from NADH/FADH2 to generate ATP via the ETC and ATP synthase.

• Fermentation: Anaerobic process regenerating NAD+ by converting pyruvate into various end products (e.g., lactic acid, ethanol).

• Preparatory Steps: Glucose must be transported into the cell and phosphorylated before glycolysis.

• Pyruvate Conversion: Before entering the Krebs cycle, pyruvate is converted to acetyl-CoA.

• Respiration Types:

  • Aerobic Respiration: Uses oxygen as the final electron acceptor.

  • Anaerobic Respiration: Uses other inorganic molecules (e.g., nitrate, sulfate) as final electron acceptors.

  • Fermentation: Organic molecules serve as both electron donors and acceptors; no ETC involved.

Microbial Growth & Reproduction

Binary Fission and Growth Phases

• Binary Fission: Asexual reproduction in bacteria, producing two genetically identical daughter cells.

• Bacterial Growth Curve Phases:

  • Lag Phase: Cells adapt to new environment; little to no division.

  • Log (Exponential) Phase: Rapid cell division; population doubles at a constant rate.

  • Stationary Phase: Growth rate slows; nutrient depletion and waste accumulation balance cell division and death.

  • Death Phase: Cells die at an exponential rate due to lack of nutrients and toxic waste buildup.

• Importance of Growth Phase:

  • Experimental reproducibility and interpretation depend on knowing the growth phase.

  • Antibiotic susceptibility is highest during the log phase when cells are actively dividing.

Microbial Metabolism Types

Oxygen Requirements and Metabolic Strategies

• Oxygen Requirements:

  • Obligate Aerobes: Require oxygen for growth.

  • Obligate Anaerobes: Cannot tolerate oxygen; grow only in its absence.

  • Facultative Anaerobes: Grow with or without oxygen, but better with oxygen.

  • Aerotolerant Anaerobes: Do not use oxygen but can survive in its presence.

  • Microaerophiles: Require low levels of oxygen.

• Growth Patterns in Oxygen Gradients: Bacteria display characteristic growth patterns in test tubes depending on their oxygen requirements.

• Metabolic Strategies:

  • Aerobic Respiration: Complete oxidation of glucose using oxygen.

  • Fermentation: Partial oxidation of glucose without oxygen, producing organic end products.

Environmental Effects on Microbial Growth

Physical and Chemical Factors Affecting Growth

• Temperature Classifications:

  • Psychrophiles: Grow at 0–20°C (cold-loving).

  • Mesophiles: Grow at 20–45°C (moderate temperatures).

  • Thermophiles: Grow at 45–80°C (heat-loving).

  • Hyperthermophiles: Grow above 80°C (extreme heat).

• Nutrient Availability: Essential for microbial growth; limitation can restrict population size.

• Osmotic Pressure:

  • High salt or sugar (hypertonic) environments can inhibit growth by causing water loss from cells.

• Biofilm Formation: Microbes form structured communities attached to surfaces, providing protection and enhanced survival.

Applied Microbiology Case Study: Water Contamination

Indicators and Environmental Factors

• Indicators of Contamination: Fecal coliforms (e.g., Escherichia coli) are used to detect water contamination by fecal matter.

• Environmental Conditions Promoting Growth:

  • Warm temperatures and nutrient-rich runoff or waste increase microbial growth.

• Freshwater vs Marine Microbial Growth: Different microbes dominate in each environment due to salinity and nutrient differences.

• Biofilms in Natural Environments: Biofilms can form on rocks, pipes, and other surfaces, impacting water quality and ecosystem health.

Microbial Control Methods

Physical and Chemical Methods of Control

• Sterilization: Complete destruction of all microbial life, including spores.

• Disinfection: Elimination of most pathogens (not spores) from inanimate objects.

• Antisepsis: Destruction of pathogens on living tissue.

• Sanitization: Reduction of microbial counts to safe public health levels.

• Physical Control Methods:

  • Moist Heat (Autoclaving): Uses steam under pressure to sterilize.

  • Dry Heat: Incineration or hot air ovens for sterilization.

  • Radiation: Ionizing (e.g., gamma rays) and nonionizing (e.g., UV) radiation for disinfection or sterilization.

  • Filtration: Physically removes microbes from liquids or air.

• Food Preservation Methods:

  • Freezing: Slows microbial metabolism.

  • Pasteurization: Reduces microbial load without sterilizing.

  • Osmotic Pressure: High salt or sugar inhibits growth by dehydration.

Laboratory Media & Techniques

Types of Media and Their Uses

• Chemically Defined Media: Exact chemical composition is known; used for fastidious organisms.

• Selective Media: Inhibits growth of some microbes while allowing others to grow.

• Differential Media: Distinguishes between different types of microbes based on their biological characteristics.

• Reducing Media: Contains chemicals that remove oxygen, supporting growth of anaerobes.

• Culturing Under Different Oxygen Conditions: Specialized media and equipment (e.g., anaerobic jars) are used to grow microbes with varying oxygen requirements.

Enzyme Structure & Function (Advanced)

Holoenzyme Components and Cofactor Roles

• Holoenzyme: The complete, active form of an enzyme, consisting of:

  • Apoenzyme: The protein portion (inactive without cofactor).

  • Cofactor: Non-protein component (e.g., metal ion or coenzyme) required for activity.

• Role of Cofactors: Assist in enzyme function, often by stabilizing substrate binding or participating in the reaction.

Antibiotics & Growth Sensitivity

Growth Phase and Antibiotic Action

• Bacteria are most susceptible to antibiotics during the log (exponential) phase, when cell wall synthesis and division are active.

• Antibiotics like penicillin inhibit cell wall synthesis, making actively dividing cells more vulnerable.

Microbial Fermentation & Food Production

Fermentation Pathways and Applications

• Fermentation Pathways: Bacteria convert sugars to various end products (e.g., lactic acid, ethanol, CO2).

• Products of Fermentation: Include acids, alcohols, gases, and other organic compounds.

• Aerotolerant Anaerobes: Do not use oxygen but can survive in its presence; rely on fermentation.

• Heterofermentative Organisms: Produce multiple end products from fermentation.

• Role in Food Production: Microbes are used to produce yogurt, cheese, sauerkraut, and other fermented foods.

Graph & Figure Interpretation

Analyzing Microbial Data

• Growth curves show population changes over time; phases correspond to physiological states.

• Oxygen requirement diagrams and test tube growth patterns help identify metabolic types.

• Matching visual data to metabolic types and growth phases is essential for interpreting experimental results.

Essay Example: Lactobacillus Metabolism

Metabolic Pathways and End Products

• Lactobacillus is an aerotolerant anaerobe and heterofermentative organism.

• It does not use oxygen for respiration but can survive in its presence.

• Metabolic pathway: Heterolactic fermentation, converting glucose to lactic acid, ethanol, and CO2.

• Organic compounds produced: Lactic acid, ethanol, and carbon dioxide.

Table: Oxygen Requirements and Growth Patterns

Type	Oxygen Requirement	Growth Pattern in Tube
Obligate Aerobe	Requires O2	Growth at top only
Obligate Anaerobe	Cannot tolerate O2	Growth at bottom only
Facultative Anaerobe	Grows with or without O2 (better with O2)	Growth throughout, more at top
Aerotolerant Anaerobe	Does not use O2, tolerates it	Even growth throughout
Microaerophile	Requires low O2	Growth just below surface

Table: Temperature Classifications of Microbes

Classification	Temperature Range (°C)	Example
Psychrophile	0–20	Pseudomonas spp.
Mesophile	20–45	Escherichia coli
Thermophile	45–80	Bacillus stearothermophilus
Hyperthermophile	>80	Thermococcus spp.